USGCRP-sponsored research continues to advance understanding of
the causes, magnitude, and consequences of climate change over
decades to centuries. The FY96 edition of Our Changing Planet
included a summary of recent key findings. Highlights of more recent
findings follow:

Measurements of global surface air temperature over the land and of
sea surface temperature over the oceans indicate that 1995 was the
warmest year since the beginning of continuous records circa 1860
(see figure). Despite the century's largest volcanic
eruption in 1991, four of the warmest years of the 140-year record
occurred during the 1990s. Proxy records from ice cores and other
long-term data archives indicate that this century is the warmest in
recent history. This evidence that temperatures are rising is further
confirmed by indications of receding mountain glaciers and rising sea
level.

Satellite data from the Stratospheric Aerosol and Gas Experiment II
(SAGE II) instrument indicate that the very substantial stratospheric
aerosol layer caused by the June 1991 volcanic eruption of Mt.
Pinatubo had been significantly thinned by early 1994, but that
aerosol levels were still nearly 10 times greater than before the
eruption, exerting a cooling influence that lasted several years. Data
from the Microwave Sounding Unit (MSU) also showed that the
stratosphere was warmed and the troposphere was cooled by the
Pinatubo eruption, thereby requiring a correction for these events
before performing a trend analysis with this data set in search of
human-induced climatic change.

Tropospheric aerosols, both from sulfur dioxide emissions from fossil
fuel combustion and from biomass burning, have been demonstrated
to have had a regional cooling influence on the climate. Model
simulations suggest that, at least near and downwind of source
regions, aerosol-induced cooling could be offsetting greenhouse-induced
warming, although the patterns of change and the
subsequent response of the global climate are complex. Inclusion of
the cooling influence of aerosols in model simulations of climate
change has improved the agreement between simulated and
observed temperature changes over the last century.

Recent improvements in the ability to compare the spatial and
temporal patterns of climate change predicted by models with
observations suggest that the geographic pattern of climate change is
more likely due to emissions of greenhouse gases and aerosols than
to natural variations in the climate. Similarly, the latitudinal and
latitudinal pattern of change is more likely to be due to human-induced
changes than to changes in natural factors such as volcanic
eruptions and solar radiation. Together, these global and regional
analyses suggest that a significant fraction of climate change over the
last century is due to human rather than natural influences.